1. Field of the Invention
The present invention relates to a voltage comparator, and more particularly to an improvement in a circuit having a hysteresis characteristic.
2. Related Background Art
In a voltage comparator, when an input voltage which is a DC voltage approaches a comparison voltage, an output of the comparator is repeatedly turned on and off and assumes an oscillation state. This is not desirable in a circuit operation.
In order to avoid such a phenomenon, the voltage comparator is usually designed to have a hysteresis characteristic as shown in FIG. 1.
FIG. 2 shows a prior art voltage comparator numeral 1 denotes a power supply terminal to which a power supply voltage is applied, numeral 2 denotes an input terminal to which an input voltage is applied, numeral 3 denotes an output terminal from which an output voltage is produced, and numeral 4 denotes a voltage source, Q.sub.1, Q.sub.2, Q.sub.A and Q.sub.9 denotes NPN transistors, Q.sub.3 and Q.sub.4 denote PNP transistors and R.sub.A, R.sub.B, R.sub.C, R.sub.3, R.sub.4 and R.sub.5 denote resistors.
In the circuit shown in FIG. 2, as the transistor Q.sub.A is turned on from its turn-off state, a comparison voltage changes from ##EQU1## and a difference therebetween serves as a hysteresis voltage.
In this circuit, relatively accurate comparison voltage and hysteresis voltage are obtained if the power supply voltage V.sub.CC is stable and constant, but if the power supply voltage varies, those voltages vary. Accordingly, this comparator is not appropriate to an application where accuracy in absolute value is required.
FIG. 3 shows another prior art voltage comparator. The like elements to those shown in FIG. 2 are designated by the like numerals and explanation thereof is omitted.
Q.sub.B and Q.sub.10 denote PNP transistors, R.sub.D, R.sub.E and R.sub.6 denote resistors and V.sub.R denotes a reference voltage.
In this circuit, when an input voltage is high, the reference voltage V.sub.R is used as a comparison voltage, and when the input voltage is lower than the reference voltage V.sub.R and the transistor Q.sub.B is turned on, the comparison voltage rises by ##EQU2## Thus, a hysteresis voltage is attained.
In this circuit, since the reference voltage V.sub.R is used, the comparison voltage is not affected by the variation of the power supply voltage V.sub.CC, but the hysteresis voltage is affected by the power supply voltage V.sub.CC and an output impedance of the reference voltage V.sub.R, as a result, the accuracy is not assured.